This invention relates to a novel block copolymer composition for use in adhesives. More particularly, it relates to linear styrene-isoprenestyrene block copolymer compositions comprised of linear polymeric blocks and adhesives made using such compositions.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as polyisoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomena allows the A--B--A rubber to behave like a conventional vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such as in adhesive formulations; as moldings of shoe soles, etc.; impact modifier for polystyrene resins and engineering thermoplastics; modification of asphalt; etc.
Different linear block copolymers exhibit different performances in adhesive formulations. Therefore, it requires some experimentation to determine the range of polymers which are useful for a particular adhesive application. For adhesive applications such as hot melt pressure sensitive adhesives, it is desirable that the polymer or the adhesive formulation containing it have the following characteristics: the viscosity of the neat polymer should range from 5,000 to 15,000 poise; the viscosity of the adhesive should be less than 20,000 poise at 43% wt. polymer content in the adhesive; the 180.degree. peel of the adhesive formulation should be greater than 6.8 pli; the holding power to steel of the adhesive formulation should be at least 500 min., and the shear adhesion failure temperature (SAFT) to Mylar should be at least 70.degree. C. The present invention provides polymers and adhesive formulations which will satisfy these performance requirements.